Illumination system

ABSTRACT

An illumination system for illuminating a spatial light modulator. An integrator rod is positioned in the light path between an arc lamp and the spatial light modulator. At the input surface of the integrated rod, there is positioned a field lens for defocusing areas of turbulence within the arc lamp. In the output surface of the integrated rod there is positioned a plate effective to prevent dust from being attracted to the output surface of the integrator rod. The integrator rod is effective to combine light from two separate light sources.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to illumination systems. In particular theinvention relates to illumination systems for producing a beam of lightfor illuminating a spatial light modulator, the spatial light modulatorproducing a spatially modulated beam of light which may be projectedonto a display screen.

2. Discussion of the Background

The spatial light modulator may take the form, for example, of a digitalmicromirror device also known as a deformable or deflectable mirrordevice (DMD). Digital micromirror devices comprise an array ofdeflectable mirror elements, each mirror element being mounted on atorsion element over a control electrode. Applying an electric fieldbetween each mirror element and the associated control electrode causesthe mirror element to pivot. Thus the direction of light reflected fromeach mirror element may be changed by application of suitable electricaladdress signals to the digital micromirror device, the electricaladdress signals usually being derived from an input video signal. Inparticular, each mirror element may be caused to reflect light either inan “on” direction towards a projector lens for projection onto a displayscreen, or in an “off” direction towards a beam dump. It is thuspossible to spatially modulate a beam of light directed onto the arrayof mirror elements, the beam being projected onto a display screen so asto produce a projected image. The pixels of the image displayed on thedisplay screen will be derived from one or more of the mirror elementsof the digital micromirror device.

In order to provide a sufficiently intense light beam to address such adigital micromirror device, an arc lamp arranged to provide asubstantially parallel output beam has been used as the light source.One example of such an arc lamp is described in our co-pending EuropeanPatent Application EP-A-0646284.

As a projection apparatus incorporating a pixellated spatial lightmodulator such as a digital micromirror device requires very uniformillumination across the array of deflectable mirror elements, it isknown to incorporate an integrator rod, also known as a light pipe, inthe light path between the light source and the array.

The operation of an integrator rod for producing a uniform beam forilluminating a spatial light modulator such as a digital micromirrordevice is illustrated schematically in FIG. 1.

In FIG. 1, a light source, for example the arc produced by an arc lampis represented by the triangle labelled 1. A condenser lens 3 iseffective to form an inverted image of the light source 1 onto the inputsurface 5 of a glass integrator rod 7 of a rectangular cross section.

Light entering the rod 7 will propagate through the rod by means ofmultiple reflections from the internal surfaces of the rod 7. The numberof reflections which the light inside the rod 7 undergoes will depend onthe angle of incidence of the light on the input surface 5 of the rod 7and the length of the rod. An even number of internal reflections of theinput light inside the integrator rod 7 will produce an inverted imageof the light source 1 in the plane of the input surface 5. An odd numberof internal reflections of the input light inside the integrator rod 7will produce a non-inverted image of the light source 1 in the plane ofthe input surface 5. Thus a large number of both even and oddreflections will lead to multiple images of the source in the plane ofthe input surface 5 of integrator rod 7, where the orientation of eachimage is determined by the number of reflections. This effectivelytransforms the original non-uniform distribution of light at the inputsurface 5 of integrator rod 7 into a more uniform light distribution atthe output surface 9 of the integrator rod 7.

The integrator rod 7 will also be effective to create an output beam ofa cross-sectional aspect ratio matched to the output face of theintegrator rod 7. This is particularly beneficial in a projectionapparatus incorporating a digital micromirror device as the circularcross-section beam produced by, for example, an arc lamp will beconverted into a rectangular cross-section beam which may be designed tomatch the aspect ratio of the digital micromirror device.

A more detailed explanation of the use of an integrator rod in anillumination system is given in “Modern Optical Engineering” by Warren JSmith, published by McGraw-Hill Book Company, 1990; pages 263 to 265.

Whilst the use of an integrator rod is particularly beneficial in aprojection apparatus incorporating a digital micromirror device, theintroduction of the integrator rod into the light path between the lightsource and the digital micromirror device may itself create opticalaberrations in the beam which illuminates the digital micromirrordevice. In particular, flicker in the light beam produced in the arclamp 1 caused by turbulent movement of the gas within the arc lamp maybe focused close to the output surface 9 of the integrator rod 7, andappear in the projected image.

Furthermore, any dust appearing on the output face of the integrator rodwill be focused by the projector lens on to the display screen.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an illuminationsystem suitable for illuminating a spatial light modulator wherein theabove disadvantages may be avoided, together with an optical componentfor use in such a system.

It is in some circumstances necessary to use two separate light sourcesto address a spatial light modulator. EP-A-0704737 discloses anillumination system for a deformable mirror device in which two separatelight sources are used to illuminate a digital micromirror device. Thisenables light of two different colours to be alternately directed to thedigital micromirror device. However, such an arrangement will suffer thedisadvantage that any spatial irregularities in the beam produced byeither of the two individual light sources will be projected directlyonto the digital micromirror device.

It is a further object of the present invention to provide anillumination system for a spatial light modulator which may include morethan one light source.

According to a first aspect of the present invention there is provide anillumination system for a spatial light modulator including a lightsource, means for imaging an image of the light source onto the inputsurface of an integrator rod, and a field lens interposed between theimaging means and the input face of the integrator rod, the field lensbeing effective to focus an image of turbulent light within the lightsource away from the output face of the integrator rod.

According to a second aspect of the present invention there is providedan illumination system for a spatial light modulator including anintegrator rod interposed in the light path between a light source andthe spatial light modulator, a protective transmissive layer beingpositioned at the output surface of the integrator rod.

According to a third aspect of the present invention there is providedan illumination system for a spatial light modulator including at leasttwo light sources, an integrator rod, and means in respect of each lightsource effective to couple light from the respective light source intothe integrator rod.

According to a fourth aspect of the present invention there is providedan illumination system for a spatial light modulator wherein at leastone of the optical components is formed with a diffractive surfaceeffective to cause light within unwanted wavelength bands to be removedfrom the light incident on the spatial light modulator.

According to a fifth aspect of the present invention there is provided aprojection device for use in a projection system including anillumination system in accordance with any one or any combination of thefirst four aspects of the invention.

According to a sixth aspect of the present invention there is providedan optical component including an integrator rod modified for use in anillumination system in accordance with any one or any combination of thefirst four aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of embodiments of the invention will now be described, by wayof example only, with reference to the accompanying drawings in which:

FIG. 1 illustrates the passage of light from a light source through anintegrator rod;

FIG. 2 is a schematic overview of a projection system including aprojection apparatus comprising a digital micromirror device and anillumination system for the digital micromirror device;

FIG. 3 illustrates the imaging of a turbulent region of the light sourceon the output surface of the integrator rod in the projection system ofFIG. 2;

FIGS. 4(a) and 4(b) are schematic illustrations of two integrator rodsfor use in an illumination system in accordance with a first embodimentof the invention;

FIG. 5 illustrates the effect of the integrator rods illustrated inFIGS. 4(a) and 4(b) on an illumination system in accordance with thefirst embodiment of the invention;

FIG. 6 is a schematic illustration of an integrator rod for use in anillumination system in accordance with a second embodiment of theinvention;

FIG. 7 illustrates an integrator rod for use in a third embodiment ofthe invention;

FIG. 8 is a schematic large scale illustration of a structureddiffractive surface used in a fourth embodiment of the invention; and

FIG. 9 is a schematic illustration of a fifth embodiment of theinvention.

FIG. 10 is a schematic illustration of an integrator rod in accordancewith the second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, the projector system to be described comprisesa projection apparatus comprising a light source in the form of a sealedbeam arc lamp 21, an aspheric condenser lens 23, an integrator rod 25,an achromatic relay lens system 27, a digital micromirror array 29 and aprojection lens system 31. The projector system further includes adisplay screen 33, onto which the projection apparatus is arranged toproject an image.

The sealed beam arc lamp 21 comprises a sealed chamber filled with agas, for example xenon, within which are mounted an anode 35 and acathode 37. It will be appreciated, however, that the positions of theanode 35 and cathode 37 may be reversed. The anode 35 and cathode 37between them define an arc gap at which an arc 39 may be struck, the arcgap being positioned at the focal point of an essentially conicreflector 41.

Further details of a suitable arc lamp are given in our co-pendingEuropean Application EP-A-0646284.

In operation of the lamp 21, light from the arc 39 is collimated by theconic reflector 41 to produce a substantially parallel output beam.

The substantially parallel output beam from the lamp 21 is focused viathe aspheric condenser lens 23 onto the input surface of the integratorrod 5 such that a radial image of the arc is formed at the input surfaceof the integrator rod 25.

The integrator rod 25 comprises a piece of optically transmissivematerial, having a rectangular or square cross-section, or isalternately a hollow tube, with reflective surfaces. All the longsurfaces of the integrator rod 25 are polished to optically flatsurfaces to achieve substantially specular total internal reflection atall the internal surfaces. Alternatively, the walls of the integratorrod 25 are coated with a reflective coating to achieve the requiredreflectivity. Thus light entering the rod 25 will propagate through therod by means of multiple internal reflections as described above inrelation to FIG. 1.

The length of the integrator rod is constrained by the required size ofthe projector system, the aspect ratio of the integrator rod, thenecessity of minimizing optical losses, and the number of reflectionsrequired to attain a uniform intensity distribution at the output face.Any suitable transmissive material with a refractive index greater thanthe surrounding medium may be chosen for the integrator rod 25. Highpurity silica is a particularly beneficial material for the integratorrod, as it has a lower refractive index (1.46) than normal glass, thusreducing transmission losses at the reflective surfaces. High puritysilica also has superior thermal characteristics when used with highintensity sources. However, glass or other transmissive materials may beused for the integrator rod.

The output of the integrator rod 25 is focussed using an achromaticrelay lens system 27 onto the mirror elements of the digital micromirrordevice 29.

It will be appreciated that whilst only one lens is shown in FIG. 2 torepresent the achromatic relay lens system 27, in reality the system 27will include a number of lenses designed to correct for geometric andchromatic aberrations introduced by the optical components within theprojection apparatus.

Each mirror element of the digital micromirror array 29 is effective toproduce spatially modulated light by directing the incident light eitheralong an “on” path through a projector lens 31 to the display screen 33,or along an “off” path to a beam dump (not shown). In order to achievethe required angular splitting between the input and output light to thedigital micromirror array, a totally internally reflective surface (notshown) effective to transmit either the incoming or reflected light tothe mirrors of the digital micromirror device, and to reflect the otherof the reflected or incoming light may be interposed in the light pathsbetween the relay lens system 27 and the digital micromirror device 29,and between the digital micromirror device and the projector lens 31.Such an arrangement is described in our International Patent ApplicationNo. WO95/22868.

Referring now also to FIG. 3, this figure illustrates the effect of theturbulent movement of the gas in the arc lamp 21. As the turbulence willcause the light from the arc 39 to be spatially modulated in acontinually varying manner within the lamp 21, dependent on the rodlength, the positions of the optical components and the focal length ofthe condenser lens, the condenser lens 23 may cause an image of thescattered light within the turbulent regions of lamp 23 to be formedjust within or beyond the output face of the integrator rod 25. Theimage of the scattered light will be relayed onto the DMD by relay lenssystem 27, this light being focused by the projector lens 31 onto thedisplay screen 33. Thus a randomly variable intensity light pattern willbe superimposed on the integrated image formed at the display screen 33.As the image at the output surface of the integrator rod is projectedvia the projector lens 31 onto the display screen 33, flicker in theprojected image caused by the turbulence within the lamp 21 will beapparent on the display screen.

First Embodiment

In order to overcome the above described problem of the imaging of theturbulent region within the lamp in the displayed image on the displayscreen 33, in accordance with the first embodiment of the invention, afield lens is positioned at the input face of the integrator rod 25 asshown in FIGS. 4(a) and 4(b). The field lens may be convex on bothfaces, or planar convex.

Referring to FIG. 5 the effect of the field lens 41 will be to focus theimage of the turbulent regions of the lamp 21 within the integrator rod.Thus light originating from the turbulent regions of the lamp 21, andindicated as a dashed line in FIG. 5, will undergo reflections withinthe integrator rod 25, thereby cancelling out some of the randomfluctuations in the image formed by the turbulence. However, thesubstantially collimated beam produced from the arc 39 within the lamp21 after reflection by the conic reflector is focused by the condenserlens 23 onto the input surface of the integrator rod 25. This light willpass essentially through the centre of the field lens 41, and willtherefore be undeviated by the field lens 41.

The result of this is that the required superimposed inverted andnon-inverted images of the turbulent region of the lamp 21 will beformed at the output of the integrator rod 25 so as to create a uniformillumination beam for the digital micromirror device 29.

Two possible arrangements for the field lens are shown in FIGS. 4(a) and4(b). In FIG. 4(a) the planar face of a planar convex lens 41 is locatedat the input surface of the integrator rod 25, and may be fused,optically coupled using a layer of oil of suitable refractive index, orcemented using a suitable optical cement to the input surface of theintegrator rod 25. The lens 41 may be separated from the integrator rodalong the optical axis though this may not be so optically efficient.Where the integrator rod is a hollow tube, the convex lens willgenerally take the form of a separate lens as shown in FIG. 4(a).

In a preferred arrangement, as shown in FIG. 4(b) a convex lens 43 isformed integrally with the input surface of the integrator rod, thusavoiding the use of optical cements with the associated optical lossesand constraints on thermal loading.

The required curvature of the field lens 41 to produce the requiredshift of the image of the turbulent regions of the lamp 21 will dependon a number of parameters including the focussing power of the asphericcondenser lens 23, the spacing of the lamp 21 to the aspheric condenserlens 23, the spacing of the aspheric condenser lens 23 to the integratorrod 25, and the refractive index of the integrator rod 21 and field lens41. The inventors have performed a number of computer simulationsvarying the various parameters to determine the optimum combination ofparameters. In one particular example these parameters have thefollowing values:

Length of integrator rod: 95 mm

Refractive index of integrator rod: 1.45856

Spacing lamp to condenser lens: 155 mm

Spacing condenser lens to rod: 63.7 mm

Curvature condenser lens on lamp side: +0.0233 mm⁻¹

Curvature condenser lens on rod side: −0.00762 mm⁻¹

Thickness condenser lens: 26.5 mm

Refractive index of condenser lens: 1.516

Radius of curvature input surface of integrator rod: +19.00 mm

Where no field lens 41 (or curvature at the input surface of theintegrator rod 25) is present, the image of the turbulent region of thelamp is focussed in the region of the output face of the integrator rod25 leading to the problems discussed above. However, where the inputsurface of the integrator rod 25 is formed with a convex curvature of+19 mm, the object conjugate corresponding to an image at the exit faceof the integrator rod 25 will be shifted to a position between the lamp21 and the condenser lens 23, at a spacing of approximately 37 mm fromthe lamp along the optical axis. Thus, as described above, lightoriginating from the turbulent regions of the lamp 21 will undergoreflections within the integrator rod 25, reducing the temporalfluctuations in the light reaching the digital micromirror device 29originating from the turbulent regions of the lamp.

Second Embodiment

A further problem of the system shown in FIG. 1 is that dust or otherdebris electrostatically or otherwise attracted to the output surface 9of the integrator rod 7 will be imaged by the achromatic relay lenssystem and projection lens system onto the display screen. Whilst it maybe possible to remove the dust by careful cleaning in dust freeenvironments, this is very time consuming and risks damage to thepolished optical surfaces.

Referring now also to FIG. 6, in order to overcome this problem, thereis provided at the output surface of the integrator rod 25 a glass plate61. The thickness of the plate 61, in the direction perpendicular to theoutput surface of the integrator rod 25, is selected so that the dust,now on the outer surface of plate 61, will be out of focus at thedigital micromirror device 29.

In use of the projection system, using the composite integrator rod 25and glass plate 61, the achromatic relay lens system 27 remains focussedon the output face of the integrator rod 25 to provide a sharp image ofthe light from the arc lamp at the required format ratio, and with auniform intensity distribution. Any dust or other debris settling on thefree surface of the glass plate 61 will not be at the object conjugateof the achromatic relay lens system 27, and hence will appear out offocus at the digital micromirror device 29 and at the display screen 33.

The lateral dimensions of the plate 61, that is the dimensions of theplate 61 in the plane parallel to the output surface of the integratorrod 25 is chosen to exceed the lateral dimensions of the output surfaceof the integrator rod 25, in particular to exceed the lateral dimensionsof the cone of light 63 emitted from the output face of the integratorrod 25.

The inventors for the present application have performed a number ofexperiments to determine the optimum dimensions of the glass plate 61.Too great a thickness reduces the light output of the integrator rod,whilst too small a thickness does not produce the required defocussingeffect. The inventors have found that a thickness in the region of 3 mmgives a particularly beneficial result.

The glass plate 61 may be either cemented or fused to the output face ofthe integrator rod in clean conditions during manufacture of theintegrator rod 25.

It will be appreciated that whilst a glass plate is described in theabove embodiment to protect the output surface of the integrator rod,the protective means could also form an imaging element for theprojection system, or a prism member effective to redirect light passingthrough the integrator rod. This is shown in FIG. 10, where theprotective means 100 could be an imaging element or a prism element.

It will be appreciated that the input surface of the integrator rod 25shown in FIG. 6 may carry a separate or integrated convex lens at theinput surface as shown in FIGS. 4(a) and 4(b).

Third Embodiment

Referring now to FIG. 7, this Figure illustrates the use of anintegrator rod to combine the outputs of two separate light sources 701,703 in order to produce a higher intensity light beam than is possiblefrom a single light source.

Each light source as before suitably takes the form of a sealed beam arclamp, but may be any other suitable high intensity light source. Theintegrator rod 705 has, at its input surface 707, two oppositelydirected prisms 709, 711. Between each prism 709, 711 and each of thetwo light sources 701, 703 there is arranged a respective condenser lens713, 715 arranged to focus the parallel beam of light emitted from therespective light source 701, 703 onto the input face 707 of theintegrator rod 705.

The light passing from each light source 701, 703 into each of theprisms 709, 711 will be totally internally reflected by the hypotenusesurface of the respective prism 709, 711 to form respective light spotson the input surface 707 of the integrator rod 705. This light will thenbe reflected within the integrator rod 705 as before the light from thetwo light sources being mixed to produce a composite image of light fromthe two light sources 701, 703 on the output surface 717 of theintegrator rod 705. Thus a uniform even illumination field will beproduced at the output surface 717 of the integrating rod 705 whichcombines the light flux from the two sources 701, 703.

It will be appreciated that whilst as described above, light from thetwo light sources 701, 703 is directed simultaneously onto the inputsurface of the integrator rod, in some circumstances it may be requiredthat light from each of the light sources 701, 703 be directedsequentially onto the input surface of the integrator rod. This may bethe case, for example, where each of the light sources is arranged todirect light of a different colour onto the digital micromirror deviceto achieve a colour projection apparatus. In such a case it is possibleto arrange for light from three or more different sources to be coupledsequentially into the integrator rod. The different sources may bederived from a multiwavelength source such as a xenon arc lamp using afilter arrangement.

Fourth Embodiment

The light output of the sealed beam arc lamp used to illuminate thedigital micromirror device will include unwanted light in theultraviolet and infra-red wavelengths. This is particularly the casewhere the lamp is filled with xenon which produces a large amount oflight in the ultraviolet and infra-red wavelengths. In order to removethis unwanted light from the light illuminating the digital micromirrordevice and thus avoid heating of the digital micromirror device, inaccordance with a further aspect of the invention the integrator rodshown in any one of FIGS. 2 to 7 may be formed with a diffractivestructured surface along its long edges effective to form a dichroicmirror surface. The dichroic mirror surface is designed such that therequired visible light is totally internally reflected within theintegrator rod, whilst the unwanted infra-red and ultraviolet lightpasses out through the long surfaces of the integrator rod to beabsorbed by a suitable beam dump (not shown) external to the integratorrod.

Details of such structured diffractive surfaces are given in thefollowing article:

Applied Optics, vol 32, pages 1154-1167 (1993), D.H. Raguin and G.M.Morris, “Anti-reflection structured surfaces for the infrared spectralregion”.

In summary, the diffractive surface is a periodically structured surfacefor example a series of ridges or an array of protuberances produced byholography, with a period typically 2 to 3 times smaller than thewavelength of infra-red light. The grating surface will thus suitablyhave a period of between0.2 microns and 2 microns. An example of such asurface is shown in FIG. 8 which shows an array of cylindricalstructures formed on the surface of the integrator rod, the structureshaving the dimensions and spacing chosen to produce the requireddichroic effect.

It will be appreciated that the use of the structured diffractivesurface is particularly advantageous in a projection apparatus inaccordance with the invention, as a dichroic coating produced by forexample thin film coating techniques, is likely to peel particularlywhere heating of the integrator rod occurs due to the high power outputbeam of the arc lamp.

A structured diffractive surface of the type described above mayalternatively or additionally be formed at the input surface (labelled 5in FIG. 1) of the rod, the lens surfaces 41, 43 shown in FIG. 4, and theouter surfaces 709 and 711 shown in FIG. 7. In such cases, thediffractive surface will be arranged to have a period such that thecoating is effective to transmit visible light and to reflect light inthe unwanted infra-red and ultraviolet wavelength bands.

Fifth Embodiment

It will be appreciated that whilst embodiments in accordance with eachof the aspects of the present invention described above can be usedseparately to provide an improved projection apparatus, a particularlybeneficial apparatus is achieved in the arrangement illustrated in FIG.9 in which corresponding components to those shown in the earlierfigures are correspondingly labelled.

As can be seen from FIG. 9, the input surfaces 801, 803 of therespective prisms 709, 711 are formed to have a convex input face, theconvex surfaces 801, 803 being treated in order to reduce reflectionlosses. These convex input faces act as the lenticular arrangement shownin FIGS. 4(a) and 4(b) and are effective to cause focussing of the lightfrom the turbulent areas of the light sources 701, 703 within theintegrator rod 705 as explained in relation to FIGS. 4 and 5.

At the output surface of the integrator rod 705 there is attached aglass plate equivalent to that shown in FIG. 6 effective to prevent dustbeing attracted to the surface 717 of the integrator rod 705 inanalogous manner to FIG. 6. This plate again may be treated in order toreduce reflection losses.

A diffractive structured surface as described in relation to the fourthembodiment and indicated as 805 in FIG. 9 is formed along the long edgesof the integrator rod 705. This coating is designed to remove infra-redand ultraviolet light from the light passing towards the output surfaceof the integrator rod as described above.

Further Modifications

It will be appreciated that whilst in each of the embodiments describedherebefore, an aspheric condenser lens is used to produce an image ofthe light source on the input surface of the integrator rod, the singlelens shown may be replaced by a combination of lenses. Furthermore, thecondenser lens may be replaced by a concave reflector positioned so asto produce the required image of the light source on the input surfaceof the integrator rod. Any of the features disclosed in relation to theembodiments of the invention described above may be incorporated in sucha system.

It will also be appreciated that whilst the invention claimed hasparticular application in an illumination system for a projectionapparatus incorporating a spatial light modulator in the form of adigital micromirror device, the invention claimed also finds applicationin illumination systems for spatial light modulators, in particularother forms of pixellated spatial light modulator systems such as liquidcrystal arrays. Whilst digital micromirror devices are reflectivedevices, and thus the projector lens system and display screen shown inFIG. 2 are positioned accordingly, it will be appreciated thatalternative spatial light modulator devices, which are transmissiverather than reflective, such as liquid crystal arrays, may be opticallyaddressed by an illumination system in accordance with the invention.

It will also be appreciated that whilst the invention finds particularapplication in an illumination system including one or more lightsources in the form of an arc lamp arranged to produce a substantiallyparallel output beam, the invention also finds application inillumination systems incorporating other forms of light sources, forexample arc lamps which produce a focused beam, or tungsten lamps.

It will also be appreciated that whilst the embodiments described by wayof example include a single digital micromirror device, a colourprojection apparatus will often include three digital micromirrordevices, each responsive to light within a different primary colourwavelength band, i.e. red, green or blue. Such apparatus will theninclude a colour splitting means, for example a pair of dichroic mirrorsin the light path between the end of the integrator rod and the digitalmicromirror devices. The spatially modulated light from the threedigital micromirror devices will then be combined prior to projection bythe projector lens 31. Alternatively, three separate colour sources maybe provided, with an integrator rod and other associated opticalcomponents being provided in respect of each primary colour channel.

What is claimed is:
 1. An illumination system for a projection systemincluding at least one spatial light modulator, the illumination systemincluding: an arc lamp; means for focussing an image of the arc which inuse is produced by the arc lamp onto an input surface of an integratorrod; and a field lens at the input surface of the integrator rod; thefield lens being effective to focus an image of scattered light withinthe arc lamp at a position within the integrator rod displaced from theoutput surface of the integrator rod.
 2. An illumination systemaccording to claim 1 wherein said imaging means comprises at least onecondensing lens.
 3. An illumination system according to claim 1 whereinsaid imaging means comprises a concave reflective surface.
 4. Anillumination system according to claim 1 wherein said field lens isintegral with the integrator rod.
 5. An illumination system according toclaim 1 wherein said field lens is attached to said integrator rod. 6.An illumination system according to claim 1 wherein said field lens isseparate from said integrator rod.
 7. An illumination system for aprojection system including at least one spatial light modulator, theillumination system including: a light source; an integrator rodinterposed in the light path between the light source and the spatiallight modulator; focussing means effective to focus the light outputfrom the integrator rod onto the spatial light modulator; and a lighttransmissive protective means at the output surface of the integratorrod effective to transmit light emerging from the integrator rod, theprotective means having a thickness such that an image of any debris onthe surface of the protective means remote from the integrator rod isnot focused on the spatial light modulator.
 8. An illumination systemaccording to claim 7 wherein the protective means has a larger area inthe direction perpendicular to the optical axis of the integrator rodthan the output surface of the integrator rod.
 9. An illumination systemaccording to claim 7 in which the protective means is a lighttransmissive plate.
 10. An illumination system according to claim 7 inwhich the protective means is an imaging element.
 11. An illuminationsystem according to claim 7 in which the protective means is a prismelement effective to redirect light passing through the integrator rod.12. An illumination system for a projection system including at leastone spatial light modulator including: at least two light sources; anintegrator rod; a prism arrangement including a non-spherical surface inrespect of each light source effective to reflect light from therespective light source onto the input face of the integrator rod suchthat the integrator rod produces a beam of substantially uniformillumination from the light incident on the rod at any one time; and afocusing lens for each light source effective to focus light from therespective light source onto a respective input surface of the prismarrangement.
 13. An illumination system according to claim 12 in whichthe light sources are arranged to direct light onto the integrator rodsimultaneously.
 14. An illumination system according to claim 12 inwhich the light sources are arranged to direct light onto the integratorrod sequentially.
 15. An illumination system for a projection systemincluding at least one spatial light modulator, the illumination systemincluding: an arc lamp; means for focusing an image of the arc which inuse is produced by the arc lamp onto an input surface of an integratorrod; and a field lens at the input surface of the integrator rod, thefield lens being effective to focus an image of scattered light withinthe arc lamp at a position within the integrator rod displaced from theoutput surface of the integrator rod, wherein at least one of the meansfor focusing the integrator rod and the field lens is formed with astructured surface constituting a diffractive element effective toreflect light within selected wavelength bands, and transmit lightwithin other wavelength bands.
 16. An illumination system for aprojection system including at least one spatial light modulator, theillumination system including: a light source; an integrator rodinterposed in the light path between the light source and the spatiallight modulator; focusing means effective to focus the light output fromthe integrator rod onto the spatial light modulator; and a lighttransmissive protective means at the output surface of the integratorrod effective to transmit light emerging from the integrator rod, theprotective means having a thickness such that an image of any debris onthe surface of the protective means remote from the integrator rod isnot focused on the spatial light modulator, wherein at least one of themeans for focusing the integrator rod and the light transmissiveprotective means is formed with a structured surface constituting adiffractive element effective to reflect light within selectedwavelength bands, and transmit light within other wavelength bands. 17.An illumination system for a projection system including at least onespatial light modulator including: at least two light sources; anintegrator rod; a prism arrangement effective to couple light from eachlight source onto the input face of the integrator rod such that theintegrator rod produces a beam of substantially uniform illuminationfrom the light incident on the rod at any one time; and a focusing lensfor each light source effective to focus light from the respective lightsource onto a respective input surface of the prism arrangement, whereinat least one of the integrator rod, the prism arrangement and thefocusing lenses are formed with a structured surface constituting adiffractive element effective to reflect light within selectedwavelength bands, and transmit light within other wavelength bands. 18.An illumination system according to any one of claims 15, 16 or 17 inwhich one of the wavelength bands is a visible wavelength band, and theother wavelength band is at least one of the ultraviolet and infra-redwavelength bands.
 19. A projection apparatus for use in a projectionsystem, the projection apparatus including at least one spatial lightmodulator and an illumination system in accordance with any one ofclaims 1, 7, 12, 15, 16, or 17 arranged to illuminate the spatial lightmodulator.
 20. A projection apparatus according to claim 19 wherein thespatial light modulator is a digital micromirror device.
 21. Aprojection system comprising a projection apparatus, including at leastone spatial light modulator, an illumination system in accordance withany one of claims 1, 7, 12, 15, 16, or 17 arranged to illuminate thespatial light modulator and a projection lens effective to directspatially modulated light from the spatial light modulator onto adisplay screen.
 22. A projection system comprising a projectionapparatus according to claim 19, a projection lens and a display screen,the projection lens being effective to direct spatially modulated lightfrom the spatial light modulator onto the display screen.
 23. Anillumination system according to claim 12 in which light directed fromthe two light sources is substantially perpendicular to the length ofthe integrator rod.
 24. A projection system according to claim 21,wherein the spatial light modulator is a digital micromirror device.